1. Field of the Invention
The present invention relates to a photoelectric conversion device, a method for manufacturing the same and an image pickup system, and in particular, to a CMOS area sensor and a method for manufacturing the same, as well as a photoelectric conversion device which can be suitably used in the manufacturing method thereof, a method for manufacturing the same and an image pickup system.
2. Related Art
A CCD has been known as a solid state image pickup element for converting image signals to electrical signals. This CCD has photodiode arrays, which are adapted to read out charge stored in each photodiode as electrical signals by applying pulse voltages to the same.
Moreover, a CMOS area sensor into which peripheral circuits are integrally formed together by a CMOS process has been used in recent years.
A CMOS area sensor has advantages such as low power consumption and low drive power compared with a CCD, and so future expansion of demand for CMOS area sensors is expected.
A CMOS area sensor will be described below with reference to FIG. 8 as a representative example of a photoelectric conversion device.
FIG. 8 shows a schematic cross-sectional view of a photodiode unit 301 and a transfer MOS transistor unit 302 of a CMOS area sensor.
Reference numeral 303 denotes an N-type silicon substrate; reference numeral 304 denotes a P-type well; reference numeral 307 denotes a gate electrode of the transfer MOS transistor; reference numeral 308 denotes an N-type charge storage region of the photodiode; and reference numeral 309 denotes a surface P-type region for providing the photodiode with a buried structure. In addition, reference numeral 305 denotes a field oxide film for element isolation; and reference numeral 310 denotes an N-type high concentration region which forms floating diffusion and functions as a drain region of the transfer MOS transistor 302.
Reference numeral 311 denotes a silicon oxide film to insulate the gate electrode and a first wiring layer; reference numeral 312 denotes a contact plug; reference numeral 313 denotes a first wiring layer; reference numeral 314 denotes an interlayer insulating film to insulate the first wiring layer from a second wiring layer; reference numeral 315 denotes a second wiring layer; reference numeral 316 denotes an interlayer insulating film to insulate the second wiring layer from a third wiring layer; reference numeral 317 denotes a third wiring layer; and reference numeral 318 denotes a passivation film.
A color filter layer (not shown) and a micro-lens for increasing sensitivity are formed over the passivation film 318.
Light incident on the surface then enters the photodiode through an opening bordered by the third wiring layer 317.
The light is absorbed in the N-type charge storage region 308 or the P-type well 304 of the photodiode to form an electron-hole pair, from which electrons are stored in the N-type charge storage region 308.
The transfer MOS transistor unit of the CMOS area sensor is preferably designed so that it can efficiently, preferably completely, transfer electrons stored in the photodiode to the floating diffusion unit.
If a pixel with low transfer efficiency is present, the output of the corresponding pixel is smaller than that of normal pixels, so that a black output image is produced, which is a defect called “a, black flaw”.
Further, the output may sometimes vary whenever a photograph is taken, causing deterioration of images as a random noise.
Furthermore, the transfer MOS transistor is connected to the photodiode, and when dark current is produced and electrons flow into the photodiode while the transfer MOS transistor is turned off, the output of corresponding images is larger than that of normal images. This may produce a white output image, which is a defect called “a white flaw”.
In order to prevent the above-described problems, prior art for the transfer MOS structure has proposed a structure in which work function of a gate electrode is controlled as described in Japanese Patent Application Laid-Open No. 2001-196572, or in which a gate electrode is additionally provided as described in Japanese Patent Application Laid-Open No. 2004-039671.
However, it has been desired that transfer efficiency of electrons in the photodiode is improved, or that preferably the electrons are completely transferred, in conventional photoelectric conversion devices, in particular in the transfer MOS transistor structure of the CMOS area sensor. In order to achieve the desire, a structure is proposed in which the signal charge storage region is extended under the transfer gate (U.S. Pat. No. 6,504,193). Further, a structure is proposed in which an n-type diffusion region (when the transfer MOS transistor is an n-MOS) is provided between a photodiode and a drain of the transfer MOS transistor (U.S. Pat. No. 6,661,459); wherein the concentration of impurities in the diffusion region had to be increased.
Moreover, for transferring charge, high voltage on the plus side had to be applied to the gate electrode of the transfer MOS transistor (in the case where the transfer MOS transistor is an N-MOS transistor).
On the other hand, electrons produced from the interface of Si and SiO2 of the MOS transistor when the transfer MOS transistor is turned off, that is, during the storage of charge, need to flow into the photodiode side. Therefore, the off-voltage with a high absolute value on the minus side has to be applied until the channel layer near the interface is sufficiently filled with holes.
The higher the concentration of impurities in the n-type region between the transfer MOS transistor and the photodiode, the more minus-side voltage has to be applied.
Consequently, in order to obtain better images, it has been necessary to apply to the gate electrode the voltages that differ greatly between the states where the transistor is turned on and off.
This requires application of high voltage to the gate electrode of the MOS transistor, which may break insulation of a gate insulating film or may deteriorate the characteristic of the MOS transistor.
In order to solve these problems, it is necessary to increase the thickness of the gate insulating film of the MOS transistor, and/or to increase the dimension of the MOS transistor elements, which have been obstacles in high-integration and miniaturization of CMOS area sensors.
Moreover, in the structure as described in Japanese Patent Application Laid-Open No. 2001-196572, work function of the gate electrode of the transfer MOS transistor is controlled by subjecting it to a special production step, which causes a problem of increase in production cost. In addition, when a gate electrode is additionally provided as described in Japanese Patent Application Laid-Open No. 2004-039671, the number of elements per pixel increases, causing a problem of preventing high-integration.
Thus, the present invention has been created to solve the above-described problems, and provides a photoelectric conversion device which increases the saturated number of charges in the photoelectric conversion device and efficiently reads out the charge obtained by the photoelectric conversion.